This invention relates to an improvement in the structure of bipolar depolarized electrode-type, filter press-type electrolysis cells. More particularly it relates to those of such cells which employ permselective ion exchange membranes, or hydraulically permeable diaphragms planarly disposed between flat surfaced, parallel, porous, depolarized anodes and/or cathodes when said anodes and cathodes are mounted at a distance from the fluid impermeable structure of the bipolar electrode which physically separates adjacent electrolysis cells. Such cells are particularly useful in the electrolysis of aqueous solutions of alkali metal chlorides; especially in the electrolysis of aqueous solutions of sodium chloride (sodium chloride brine). The cell structure may also be used in electrolyzing other solutions to make products such as potassium hydroxide, iodine, bromine, bromic acid, persulfuric acid, chloric acid, adiponitrile and other organic compounds made by electrolysis.
It is well established that various chemicals can be produced in electrochemical cells containing an anode and a cathode. For example, alkali metal chlorates, such as sodium chlorate, have been formed electrolytically from a sodium chloride brine in cells without a separator positioned between the anode and the cathode.
When a separator, such as a hydraulically permeable asbestos or polytetrafluoroethylene diaphragm or a substantially liquid impervious ion exchange membrane, is used in a cell to electrolyze a sodium chloride brine, the electrolytic products will normally be gaseous chlorine, hydrogen gas, and an aqueous solution containing sodium hydroxide.
For a number of years gaseous chlorine was produced in electrolytic cells wherein an asbestos diaphragm was interposed between finger-like, anodes and cathodes which were interleaved together. During the past several years it has become apparent that the use of a substantially liquid impermeable cation exchange membrane may be preferable to the more well established diaphragm in instances where a higher purity, for example a lower sodium chloride content, higher sodium hydroxide product is desired. It was found to be more convenient to fabricate ion exchange type electrochemical cells from relatively flat or planar sheets of ion exchange membrane rather than to interleave the membrane between the anode and cathode within the older finger-like cells used with asbestos diaphragms.
The newer, so-called flat plate electrochemical cells using a planar piece of ion exchange membrane to separate the anolyte from catholyte compartments also have a plurality of solid, liquid impervious frames adapted to support the anode on one side and the cathode on the opposite side. These frames have previously been constructed of materials such as metal and plastic, but neither of these materials has been found to be entirely satisfactory. In any electrochemical cell, including both monopolar and bipolar cells, there is a possibility that electrolyte may leak from within the cell to the exterior. In instances where such leakage has occurred in cells with iron or other ferrous type frames, it was found that the iron frame corroded or was itself electrolytically attacked. Plastic frames are not generally subject to the electrolytic attack, but are normally not resistant to the anolyte and/or catholyte within the cell under operating conditions for extended periods of time, for example, several years.
It is desired to provide a structural frame for use in electrochemical cells which would minimize the corrosion problems and would increase the relatively short useful life attendant with those frames used by the prior art.
The cell of the present invention is particularly well suited for use with depolarized electrodes. Such electrodes may take the form of depolarized cathodes, where an oxygen-containing gas is contacted with one side of the cathode and an electrolyte is contacted with another side of the cathode. The cathode is a porous body which allows both the gas and the electrolyte to enter. Inside the cathode, with the addition of electrical energy from a power supply, electrochemical reactions are caused to occur. In the case of chlor-alkali cells, hydroxyl ions are produced without the production of hydrogen.
In the case of depolarized anodes, a hydrogen-containing gas is contacted with one side of an anode and an electrolyte is contacted with another side of the anode. The anode is porous and allows both the gas and the electrolyte to enter. Electrochemical reactions are caused to occur within the internal portions of the anode. In the case of chlor-alkali cells, hydrogen gas may be contacted with one side of the anode and a sodium chloride brine solution contacted with another side of the anode, to produce HCl.
Patents which teach various type of depolarized cathodes includes: U.S. Pat. Nos. 4,187,350; 4,213,833; 4,224,129; 4,256,545; 4,260,469; 4,269,691; 4,312,720; 4,317,704; 4,341,606; 4,406,758; 4,445,896; European Patent Application Nos. 0,051,432; 0,051,435; 0,051,437 and 0,051,439.
References which include depolarized anodes include: U.S. Pat. Nos. 3,124,520; 4,447,322; European Patent Application No. 107,612-A and "An Electrochemically Regenerative Hydrogen-Chlorine Energy Storage System", D. T. Chin, R. S. Yeo, J. McBreen, S. Srinivasan, Journal of Electrochemical Society, Volume 126, page 713, 1979.